The present invention relates to an electrode structure for a semiconductor device mounted on a circuit board and a method for forming the electrode structure, and a mounted body including the semiconductor device, and more particularly to an electrode structure for a semiconductor device mounted on a circuit board in the face-down state and a method for forming the electrode structure, and a mounted body including the semiconductor device.
Conventionally, a semiconductor device is often mounted on a circuit board by soldering. In recent years, however, the package for the semiconductor device has been small-sized and the number of connecting terminals has been increased. Consequently, the space between the connecting terminals has been reduced. Thus, it has become gradually harder to apply the soldering technique according to the prior art.
A method for directly attaching the semiconductor device onto the circuit board to reduce the mounting area so that the circuit board can be used efficiently has been proposed. In particular, the following mounting method is considered effective because the mechanical strength obtained after connecting the semiconductor device to the circuit board is great and direct connection can be performed (for example, xe2x80x9cMicroelectronics Packaging Handbook; Nikkei BP published on Mar. 27, 1991xe2x80x9d). According to the above-mentioned method, a semiconductor device having an electrode structure is manufactured in which a deposited film such as an adherence metal or a diffusion preventive metal is formed on an aluminum electrode in advance when connecting the semiconductor device to the circuit board and a bump electrode made of a solder is formed on the deposited film by a plating method, and the semiconductor device is provided on the circuit board in the face-down state and heated at a high temperature so that the solder (bump electrode) and the terminal electrode of the circuit board are fused.
According to this method for forming the bump electrode, the electrode structure is complicated so that a lot of steps are necessary. For this reason, a method for easily forming the bump electrode on the aluminum electrode of the semiconductor device by using the wire bonding technique has been proposed (for example, Japanese Unexamined Patent Application No. 49-52973).
An example of the electrode structure for a semiconductor device and a method for forming the electrode structure and the mounted body including the semiconductor device according to the prior art will be described below with reference to the drawings. FIG. 3 is a sectional view showing the electrode structure for the semiconductor device according to the prior art. FIG. 4 is a sectional view showing the mounted body including the semiconductor device according to the prior art.
As shown in FIG. 3, an aluminum electrode 11 is formed on an IC substrate 10. A passivation film 13 is formed on the IC substrate 10 so as to cover the peripheral portion-of the aluminum electrode 11. A bump electrode 12 is formed on the aluminum electrode 11 by a wire bonding method.
As shown in FIG. 4, the semiconductor device having the bump electrode 12 is aligned in the face-down state in such a manner that the bump electrode 12 closely comes in contact with a terminal electrode 15 of a circuit board 14, and is provided on the circuit board 14. The bump electrode 12 and the terminal electrode 15 are fused together by thermo-compression bonding or ultrasonic welding. Thus, a mounted body including the semiconductor device is obtained.
However, the semiconductor device having the above-mentioned electrode structure and the mounted body including the semiconductor device have the following problems.
More specifically, the aluminum electrode is exposed around the bump electrode. For this reason, if water exists in the exposed aluminum electrode portion, there is a possibility that aluminum flows out in a comparatively short time so that the semiconductor device is not electrically conducted to the circuit board. In order to enhance the reliability of a junction, it has been proposed to fill a gap between the semiconductor device and the circuit board with an insulating resin. In this case, however, there is a possibility that impurity ions such as chlorine contained in the insulating resin promote the corrosion of the aluminum electrode so that the semiconductor device is not electrically conducted to the circuit board. In the case where an organic board such as a glass epoxy board is used for light weight and low cost, the corrosion of the aluminum electrode becomes more serious because the organic board is generally permeable to water and a lot of impurity ions are present in the organic board.
In order to solve the above-mentioned problems in the prior art, it is an object of the present invention to provide an electrode structure for a semiconductor device and a method for forming the electrode structure and a mounted body including the semiconductor device in which the semiconductor device can be easily connected to a circuit board reliably.
To accomplish the above-mentioned object, the present invention provides an electrode structure for a semiconductor device which is suitable for mounting on a circuit board in the face-down state, comprising a bump electrode formed on an aluminum electrode of the semiconductor device; and an aluminum oxide film sufficient for the prevention of corrosion of the aluminum electrode, which is formed on a surface of the aluminum electrode exposed around the bump electrode. According to the electrode structure for the semiconductor device, the aluminum electrode is not corroded due to water and impurity ions when mounting the semiconductor device on the circuit board.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the aluminum oxide film for the prevention of corrosion should be obtained by further oxidizing a natural oxide film on the surface of the aluminum electrode.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 5 to 20% of the thickness of the aluminum electrode. According to the preferred example, a practical electrode structure for the semiconductor device can be obtained. In other words, the thickness of the aluminum oxide film which is smaller than 5% of the thickness of the aluminum electrode is not practical because it is very hard to form the aluminum oxide film densely. Furthermore, the thickness of the aluminum oxide film which is greater than 20% of the thickness of the aluminum electrode is not practical because the resistance value of the aluminum electrode portion is increased too much.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 0.05 to 0.2 xcexcm. According to the preferred example, a practical electrode structure for the semiconductor device can be obtained. In general, the thickness of the aluminum electrode is about 1 xcexcm. Consequently, the aluminum oxide film having a thickness of 5 to 20% of the thickness of the aluminum electrode can be obtained.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the bump electrode should be made of Au. According to the preferred example, the surface of the bump electrode is not oxidized at the step of forming an oxide film on the aluminum electrode of the semiconductor device. Accordingly, the bump electrode made of Au is convenient for the following bonding step. The reason is that if an oxide film is formed between the bonding layer and the surface of the bump electrode, a good electrical conduction cannot be obtained.
The present invention provides a method for forming an electrode structure for a semiconductor device which is mounted on a circuit board in the face-down state, comprising the steps of forming a bump electrode on an aluminum electrode of the semiconductor device; and forming an aluminum oxide film sufficient for the prevention of corrosion on a surface of the aluminum electrode that is exposed around the bump electrode. According to the method for forming the electrode structure for a semiconductor device, it is possible to obtain a semiconductor device in which the aluminum oxide film for the prevention of corrosion is formed on the surface of the aluminum electrode. As a result, the aluminum electrode is not corroded due to water and impurity ions when mounting the semiconductor device on the circuit board.
In the method for forming the electrode structure for a semiconductor device according to the present invention, it is preferable that the bump electrode should be formed by a wire bonding method using a Au wire. For example, the bump electrode is formed by the wire bonding method in the following manner. First of all, the tip of the Au wire is fused by thermal energy to form a Au ball. Then, the Au ball formed on the tip of the Au wire is put on the aluminum electrode. Thereafter, a pressure of about 50 g is applied to the Au ball formed on the tip of the Au wire at a temperature of about 150xc2x0 C. so as to be bonded onto the aluminum electrode by pressure. Finally, the Au wire is pulled upward. After the above-mentioned steps, the bump electrode made of Au is formed on the aluminum electrode. According to the preferred example, the bump electrode can be formed directly on the aluminum electrode (a general electrode material) of the semiconductor device. Consequently, the bump electrode can be formed easily on the general purpose semiconductor device.
In the method for forming the electrode structure for a semiconductor device according to the present invention, it is preferable that the aluminum oxide film should be formed by exposing the semiconductor device having the bump electrode formed thereon to the high-temperature state. According to the preferred example, the aluminum oxide film can be formed efficiently in a short time. In this case, the high-temperature state should have a temperature of 200 to 300xc2x0 C. According to the preferred example, this temperature range does not have a bad influence on the semiconductor device itself, that is, disconnection of an internal wiring or the like. Consequently, the aluminum oxide film can be formed efficiently in a short time. Furthermore, it is preferable that the aluminum oxide film should be formed simultaneously at the step of forming the bump electrode at a temperature of 200 to 300xc2x0 C. According to the preferred example, the steps of forming the electrode structure for the semiconductor device can be simplified.
In the method for forming the electrode structure for a semiconductor device according to the present invention, the aluminum oxide film should be formed by immersing the semiconductor device having the bump electrode formed thereon in ammonium persulfate or hydrogen peroxide. According to this preferred example, the aluminum oxide film can be formed efficiently in a short time.
The present invention provides a mounted body including a semiconductor device, comprising a circuit board having a terminal electrode and a semiconductor device mounted on the circuit board in the face-down state, a bump electrode formed on an aluminum electrode of the semiconductor device and an aluminum oxide film sufficient for the prevention of corrosion that is formed on the surface of the aluminum electrode exposed around the bump electrode, the bump electrode being electrically connected to the terminal electrode on the circuit board through a bonding layer. With the present mounted body including a semiconductor device, the aluminum electrode is not corroded due to water and impurity ions. As a result, there is no possibility that the semiconductor is not electrically conducted to the circuit board. Consequently, a mounted body including a semiconductor device having high reliability can be obtained.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the circuit board should be formed from a board including an organic material. According to the preferred example, the following functions and effects can be obtained. Recently, a board containing an organic material such as a glass epoxy board, a paper phenol board or the like has been used as a circuit board for light weight and low cost. This kind of board is permeable to water and contains a lot of impurity ions therein. However, the aluminum oxide film for the prevention of corrosion is formed on the surface of the aluminum electrode that is exposed around the bump electrode. Consequently, the aluminum electrode is not corroded due to water and impurity ions. According to the preferred example, it is possible to prevent the corrosion of the aluminum electrode while obtaining a reduction in weight and cost of the semiconductor device.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the aluminum oxide film for the prevention of corrosion should be obtained by further oxidizing a natural oxide film on the surface of the aluminum electrode.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the bonding layer should be made of a conductive adhesive. The conductive adhesive is obtained by mixing an organic binder, which contributes to adhesion, and a conductive filler, which contributes to electric conduction. An epoxy resin, a polyimide resin and the like are used as the organic binder. Ag, Cu, Au, Pd, Ni and the like having a mean volume particle diameter of 2 to 5 xcexcm are used as a conductive filler. In the case where the epoxy resin is used as the organic binder, the content of the conductive filler is 60 to 70% by weight. An amine hardening agent or a phenol hardening agent is used. In the case where the polyimide resin is used as the organic binder, the content of the conductive filler is 60 to 90% by weight and the hardening temperature is about 200xc2x0 C. According to the preferred example, the thermal stress on the junction caused by the mismatching of physical property values (in particular, coefficients of thermal expansion) of the semiconductor device and the circuit board is relaxed. Thus, the mounted body including a semiconductor device can be obtained with high reliability. The reason is that the conductive adhesive has excellent relaxation ability in stress because it is generally softer than an inorganic material such as a solder.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the bonding layer should be made of a solder. According to the preferred example, the semiconductor device can be mounted at the same steps as the mounting of parts other than the semiconductor device, for example, a chip resistor, a chip capacitor and the like. As a result, the semiconductor device can also be mounted at the reflow soldering step together with other parts than the semiconductor device. Consequently, the process can be simplified.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that a gap between the semiconductor device and the circuit board should be filled with an insulating resin. An example of the insulating resin is an epoxy resin. According to the preferred example, the junction of the semiconductor device and the semiconductor device itself can be protected. In particular, the influence of water under high humidity conditions, such as the degradation of the junction or the failure of the semiconductor device, can be eliminated.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 5 to 20% of the thickness of the aluminum electrode.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 0.05 to 0.2 xcexcm.
In the structure for the mounted body including a semiconductor device according to the present invention, it is preferable that the bump electrode should be made of Au.
The present invention provides a semiconductor device suitable for mounting on a circuit board, comprising an aluminum electrode; a bump electrode formed on the aluminum electrode; and an aluminum oxide film sufficient for the prevention of corrosion of the aluminum electrode, which is formed on a surface of the aluminum electrode exposed around the bump electrode.
According to the electrode structure for the semiconductor device of the present invention, the aluminum electrode is not corroded due to water and impurity ions when mounting the semiconductor device on the circuit board.
According to the method for forming the electrode structure for the semiconductor device of the present invention, the semiconductor device in which the aluminum oxide film is formed on the surface of the aluminum electrode is obtained. As a result, the aluminum electrode is not corroded due to water and impurity ions when mounting the semiconductor device on the circuit board.
According to the mounted body including the semiconductor device of the present invention, the aluminum electrode is not corroded due to water and impurity ions. As a result, possibility that the semiconductor device is not electrically conducted to the circuit board is greatly reduced. Consequently, it is possible to obtain the mounted body including the semiconductor device which is very stable and reliable.